I do think that using the Sigma 35mm as a reference is a little bit risky. I mean, if you have a great copy never sell it, but it doesn't talk google long to find lots and lots of people have inconsistent AF, and I was one of them. It's no point for a lens to have that fantastic IQ, which it really does have, when it can't deliver that IQ when you need it to. Not even certain if you take 8-10 shots. It should be possible to buy ten 35 lenses and have them perform VERY similar, that is not the case now.
The worst part is that it might be hard to tell the difference of a poor copy or if the Sigma just works like that. If you buy one and it doesn't seem right and you exchange for three-four others and they are the same with a bit too many missed shots, is it a design flaw or all poor copies, how can you tell? All the happy AF owner I read about can use one shot or Live view for all I know..